Conventionally, some refrigeration cycle devices for vehicle air conditioners use a variable displacement compressor that compresses refrigerant by being driven by power transmitted from a vehicle engine via a drive belt. The variable displacement compressor can continuously change a refrigerant discharge capacity from 100% to about 0%.
In such a refrigeration cycle device, the discharge capacity of the compressor is decreased to about 0%, so that the compressor is brought substantially into an operation-stop state. Thus, the compressor has a clutch-less structure without using a clutch for interrupting a transmission route to the engine.
In this case, however, the drive belt may be broken by the power from the engine while the compressor is locked. Thus, a pulley has been proposed which includes a torque limiter for protecting the drive belt by interrupting the transmission of power from the engine to the compressor when a torque transmitted from the engine to the compressor reaches a predetermined value (see, for example, JP-A-10-292858).
A refrigeration cycle device is provided with an engine revolution sensor for detecting an engine speed, and a compressor revolution sensor for detecting the number of revolutions of a compressor, thereby to detect a locked state of the compressor based on a sensor signal from the engine rotation sensor and a sensor signal from the compressor rotation sensor (see, for example, JP-A-61-275017).
Another refrigeration cycle device is provided with a temperature sensor for detecting the temperature of a compressor body in addition to the above-described two revolution sensors so as to prevent incorrect determination due to a variation in number of revolutions of a compressor in an overload state caused by liquid compression or the like (see, for example, JP-A-7-232544). In this case, the locked state of the compressor is detected by using sensor signals from the two revolution sensors and the detected temperature of the compressor body.
The techniques as disclosed in the above-described JP-A-61-275017 and JP-A-7-232544, however, require not only the engine revolution sensor, but also the compressor revolution sensor so as to detect the locked state of the compressor. That is, the compressor revolution sensor which is not generally mounted on a vehicle is required in order to determine whether a torque limiter interrupts the transmission of power from the engine to the compressor.